The present invention relates to pneumatic conveying systems and in particular to a material blending system in combination with a pneumatic conveying system. The invention while not intended to be limited thereby is particularly applicable for use in achieving a homogeneous quantity of material such as pellets, powders, and granular material.
Prior to the present invention, material blenders were known including blenders for plastic pellets. Typical prior art blenders for solid particulate material include gravity type blenders wherein a plurality of withdraw tubes are mounted within a bin and material is withdrawn from the bin through the tubes which have openings at various levels within the bin. The withdrawn material is supplied to a conveying line where it is usually recirculated back into the top of the bin for withdraw a subsequent time. After a given number of recirculation cycles, the material is sent to a use point. Typical of such blenders is that shown in U.S. Pat. No. 3,351,326.
Another type of blender includes a bin or silo with an aerated bottom wherein material to be blended is supplied to the bin through an inlet in the top, a gas permeable silo bottom has air supplied therethrough to aerate material within the bin or silo. One section of the bin bottom is supplied with higher pressure air to fluidize material above that section so that material above that section spills onto other portions of the silo. Material in the aerated sections is drawn into the fluidized section. The section which is fluidized is changed in cycles. A typical such apparatus is disclosed in U.S. Pat. No. 2,844,361.
A further type of blender is a column blender such as that shown in U.S. Pat. Nos. 3,648,985 and 3,729,175. In this type of apparatus, a hollow open-ended column is mounted centrally within a bin. The bin is provided with a top material inlet and a gas permeable bottom for aerating or fluidizing material within the bin. Higher pressure air is supplied into the bottom of the column to draw material in the bin into the column for movement up through the column in the manner of an air lift for dispersal onto the top surface of the material in the bin. While such apparatus has an advantage of blending pelletized material, it has a disadvantage that the blending takes a substantial period of time, the means for supplying the material to be blended to the bin is not fully utilized and high energy is required to achieve the blending of material.
A further type of blender for pulverized material is shown in U.S. Pat. Nos. 3,148,864 and 3,148,865 wherein material is pneumatically conveyed into the bottom of a bin and the action of the material entering the bin serves to blend material already in the bin with material being supplied thereto. The apparatus and method disclosed by these two patents is particularly useful for very fine grained material such as talc. The apparatus shown in these two patents has not achieved wide acceptance and requires several feed points for the material into the bottom of the bin.
A blender which combines some of the features of the prior art is shown in U.S. Pat. No. 3,276,753 wherein a device is shown which is capable of being filled with material to be blended from either a top fill point or a bottom fill point which is directly connected to an air lift column mounted in the material bin. With that device it was considered that some blending occurred in the filling of the bin, it was apparently considered necessary to have withdrawal from several points and external recirculation of material to achieve adequate blending.
By the present invention, Applicants have provided a blending apparatus and system which is capable of combining the advantages of several of the prior art blenders and is able to do so at an economical cost. With the present invention a bin already in existence which is presently being used for storage can be converted by an economical means to a blending system which is particularly adapted for use with plastic pellets such as polyethylene pellets. The invention is also suitable for all solid particulate material including but not limited to cement raw meal, powders, sand and the like.
The present invention has several advantages over the prior art blenders in that is utilizes the conveying system energy to not only supply material to the storage bin but also to provide continuous blending of the material from the beginning of the filling cycle to the end of the filling cycle to thereby cut down on production time and save energy. The blender of the present invention saves substantial capital investment by a simple construction of a single blender column because material is supplied to the bottom of the blender and removed from the bottom of the blender. Less piping is required when compared to prior art devices where material is supplied to the top of the blending bin. The blender can eliminate the external piping associated with a tube type gravity blender when required for material recirculation. While internal circulation of materials is contemplated by the present invention, blending internally results in less degredation of product when compared to external recirculation.
With the blender of the present invention, a means is provided for supplying a booster or fluidizing gas to the bottom of the bin to not only improve blending capability but also to serve to dedust or clean the material and remove or vent gas which may be trapped within the bin or vessel. Boost gas can also be used to aid in the transfer of material to other destinations after blending is completed. The boost gas or fluidizing gas also assists in the uniform flow of material, particularly during the withdrawal of material from the bin.